Stress grading coatings for insulators

ABSTRACT

A stress grading coating for insulators comprising a non-linear semiconducting coating, such as silicon carbide, or the like using a glass binder having a fusion temperature of less than 850*C.

United States Patent [191 Hirayama Feb. 12, 1974 STRESS GRADING COATINGSFOR INSULATORS [75] lnventor:

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Feb. 4, 1972 21 Appl. No.: 223,540

[52] US. Cl 117/201, 106/44, 117/125, 174/140 R [51] Int. Cl. I-IOlb17/42 [58] Field ofSearch117/201, 215, 70 A, 70 B, 125;

106/44, 46; 174/140 C, 140 R, 110 R; 252/500, 516, 518, 520

[56] References Cited UNITED STATES PATENTS 2,590,893 4/1952 Sanborn117/125 Chikara l-Iirayama, Murrysville, Pa.

Harrington et a1 106/44 Horton 117/125 FOREIGN PATENTS OR APPLICATIONS532,117 1/1941 Great Britain 174/127 Primary Examiner-Ralph S. KendallAssistant Examiner-Michael W. Ball Attorney, Agent, or FirmH. G. Massung[57] ABSTRACT A stress grading coating for insulators comprising anon-linear semiconducting coating, such as silicon carbide, or the likeusing a glass binder having a fusion temperature of less than 850C.

11 Claims, 4 Drawing Figures 816 a GLASS PORCELAIN PAIENIEW 1-2574PORCELAIN GLAZE *xmm sic a GLASS FPORCELAIN FIG. 3

' sic a GLASS GLAZE PORCELAIN FIG.

1 STRESS GRADING COATINGS FOR INSULATORS BACKGROUND OF THE INVENTIONThis invention relates to the application of a nonlinear stress gradingcoating comprising non-linear semiconducting particles bonded with aninorganic glass onto a high voltage insulator made of ceramic or resinmaterial to prevent corona and lessen the possibil ity of a flash-over.The non-linear semiconductor particles may be silicon carbide (SiC),ferrous oxide (FeO), ferrie oxide (Fe O or titanium dioxide (TiO whichwhen slightly reduced by heating in the absence of oxygen becomes asemiconductor.

High voltage insulators must be made from material having a highdielectric strength, such as ceramics or resin material, but asemiconducting layer over an insulator surface can be used to controlvoltage stress for improved insulator performance. In the case of anuncoated insulator, voltage distribution across the insulator is notuniform. Thus, even at normal system voltages, partial breakdown mayoccur at some point on the insulator surface. These discharges cangenerate radio interference voltages. A semiconducting layer appliedover the insulator can prevent these undesirable breakdowns.

Stress grading coatings are often used on high voltage insulators toprevent corona, flashover, or radio interference voltages. Materialspresently used for coating the insulator surfaces are silicon carbidepaints, or the like, incorporating an organic resin. It is advantageousto use siliconcarbidefor this purpose due to its desirable non-linearresistance characteristics which allow the paint to behave moreefficiently as a stress grading medium. The resistance of the siliconcarbide layer varies as a function of the applied electric field, as theelectric field increases the resistance of the silicon carbide layerdecreases.

An important application of high voltage insulators is on outdoorapparatus, thus, the coating applied must have a good resistance toweather and be impervious to moisture. High pollution concentration insome areas further aggravates the deteriorative effects of weather. Incertain other applications, such as an insulator bushing used on atransformer, the stress grading paint is exposed to organic material,such as insulating oil, which has a tendency to attack the paint. Thisis undesirable since the binder in the silicon carbide paint has acertain amount of solubility in the organic transformer oil, and thiswill have an adverse effect on the dielectric strength of thetransformer oil. It is therefore desirable to have a stress gradingcoating which is resistant to organic liquids and vapors and which isalso moisture resistant.

A stress grading coating which is entirely inorganic in nature and isfired onto the ceramic surface of the bushing is resistant to organicliquids and vapors and is also moisture resistant. However, the use ofinorganic coatings in prior art applications have not been entirelysatisfactory. In prior art applications, such as described in US. Pat.No. 3,389,214 issued June 18, 1968 to W. J. Smothers, which utilizesinorganic stress grading coatings comprising silicon carbide thecoatings are applied to the insulator and fired at around 1,150C for 2to 5 hours. For economic reasons, the stress grading coatings and thegreen insulator are usually fired to gether. Firing at this hightemperature for this period of time can have an adverse effect on thestress grading coating comprising silicon carbide. At this temperature areaction can occur between the glaze and the silicon carbide, whenfired, causing bubbles or blisters to form which distort the surface ofthe insulator. Another disadvantage of the high firing temperature isthat it can cuase deterioration of the non-linear resistance propertiesof the silicon carbide coating.

SUMMARY OF THE INVENTION Percent composition by weight PbO 22.0 A1 0;,6.6 8,0;, 65.3 MgO 2.6 LiF 3.4

B 0 54.2 A1 0; l 1.5 BaO 34.3

These binder glasses result in reproducible non-linear semiconductingcoatings, whereas most other glasses adversely effect the non-linearproperty of the silicon carbide layer. Glass binder 1 has a fusiontemperature of approximately 650C, while glass binder 2 has a fusiontemperature of approximately 800C. The glass particle size isapproximately 325 mesh. The silicon carbide powders used have a particlesize of between 400 and 600 mesh. Larger silicon carbide particle sizeswere tried but resulted in poor reproducibility and low breakdownvoltages. The ratio of silicon carbide to glass is between 40-70 partsby volume of glass. Coatings containing less than 30 parts by volume ofglass were also evaluated, but it was found they formed uneven coatingswith poorly reproducible resistancevoltage characteristics.

The silicon carbide particles and glass are suspended in a solutionconsisting of 2 percent nitrocellulose in butyl acetate and mixed by asuitable means, such as a ball mill, for approximately 15 minutes, toprepare the mixture for application to the insulator. This solution isthen applied to the insulator surface by a suitable means, such asdipping or spraying, and allowed to air dry into a hard surface whichmay be readily handled. Upon firing the organic binder is burned off andthe glass melts and binds the non-linear semiconducting particles, suchas silicon carbide. The silicon carbide and glass coating is fired ontothe ceramic insulator at a temperature of less than 850C forapproximately 15 minutes. The silicon carbide and glass then form asmooth finish, impervious to moisture, which helps to keep the outersurface of the insulator clean.

The silicon carbide and glass coating can be applied over the glazedsurface or to the unglazed surface of a fired insulator. The thicknessof the fired-on coating is usually between, approximately, 4-10 mils.The coatings are uniform and tightly adherent to both the glazed andunglazed surfaces. The non-linear resistancevoltage stresscharacteristic of the glass bonded silicon carbide coating varies withthe ratio of glass used in the coating and with the grain sizes of thesilicon carbide. For example, the resistance of coatings containing 600mesh silicon carbide are higher than those containing larger particlesizes with the same volume percent of glass. It has also been determinedthat the resistance of coatings containing 600 mesh silicon carbide and60 percent glass is about an order of magnitude higher than a coatingcontaining 600 mesh silicon carbide and 40 percent glass. It has alsobeen noted that the coatings applied to the glazed surfaces have higherresistances than those applied to unglazed ceramic insulators.

By selecting the proper silicon carbide to glass ratio and the propersilicon carbide particle size, it is possible to get a coating which hasa voltage resistance characteristic very similar to those obtained insilicon carbide organic paints currently being used as stress gradingcoatings. When the coating is applied to the ceramic insulator, it isfired in an atmosphere which is reducing, oxidizing or neutral at lessthan 850C for approximately minutes. For some applications where theceramic insulator may be exposed to severe weather conditions anadditional protective layer of glaze may be applied over the siliconcarbide and glass coating.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages of the invention willbe apparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

7 FIG. 1 is a partial view in side elevation of a bushing structureembodying the features of the invention;

FIG. 2 is an enlarged section on the line II-II of FIG.

FIG. 3 is a view similar to that of FIG. 2 of a second embodiment of theinvention; and

FIG. 4 is a view similar to that shown in FIG. 2 of a third embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawings,'and FIG. 1 in particular, there is shown an insulator bushing10 comprising an insulating weatherhousing or body portion 12, anelectrically conducting stud l4 and a metallic flange 16. The bushingstud 14 and the bushing top 18 are at a high voltage and the metallicflange 16 is usually at ground potential so that a voltage gradient ispresent across the insulator 12. It is intended that the body or casing12 may be constructed from insulating material, such as ceramic orresinous materials. The voltage distribution over the surface of theinsulator body 12 is not uniform. The top portion of the body 12 nearthe bushing top 18 is more highly stressed than the portion near thegrounded metallic flange 16. When a nonlinear semiconducting coating,whose resistance is an inverse function of the applied voltage, isdisposed over the surface 20 of the insulating body 12 the voltagedistribution over the insulating body 12 is more uniform.

FIG. 2 shows the non-linear semiconducting coating of the presentinvention applied to the surface 20 of the insulator 12 so that thevoltage distribution over the surface 20 is more uniform to reduceflashover, corona, and radio interference voltage. FIG. 2 shows aportion of the insulator l2 coated with silicon carbide using aninorganic glass binder. The resistivity of this coating may be alteredby varying the thickness of the coating, the silicon carbide to glassratio, or the silicon carbide particle size. With the proper parametersthe voltageresistance characteristics of this coating can be made verysimilar to those obtained for silicon carbide stress grading paintswhich are presently in use.

FIG. 3 shows a second embodiment of the invention for use on aninsulating bushing 10 which may be exposed to severe weather conditions.This embodiment is also useful where the effects of weather areaggravated by severe damp or polluted conditions. As seen in FIG. 3, aprotective coating of glaze is applied over the silicon carbide andglass layer which is applied to the porcelain surface 20.

FIG. 4 shows a third embodiment of the invention. The silicon carbideand glass layer is applied over a glaze which is applied to theporcelain surface 20. This embodiment of the invention is useful whenglazed insulators are kept in stock or when the insulators as receivedfrom a supplier are already glazed and it is later desired to coat theinsulator with a non-linear semiconducting coating.

The apparatus embodying the teachings of this invention has severaladvantages. For example, this type of coating is especially desirable onoutdoor apparatus where the porcelain bushings 10 with a surface 20 isexposed to atmospheric deterioration. Another advantage of thisnon-linear stress grading coating is that it is entirely inorganic innature and is not susceptible to attack by the organic insulatingmaterials around which it may be used. Another area where an inorganicbonded silicon carbide layer can be useful is on a system where anyoutgassing from the coating will be undesirable, such as vacuuminterrupters. Another advantage is that this layer is resistant tomoisture. This property is important for outdoor application of thestress grading. Another distinct advantage of this inorganic coating isthe lower firing temperature as compared with the prior art inorganiccoatings. This lower firing temperature of less than 850C is a distinctadvantage since it lessens the deterioration of the non-linearproperties of the silicon carbide. Due to lower firing temperature, thenon-linear coating is reproducible and does not interact adversely withthe porcelain or the glaze.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit and scope of the invention, it is intended that allmatter contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

I claim:

1. An electric insulator comprising an insulating body, a ceramiccoating fired onto said insulating body, said ceramic coating comprisingnon-linear semiconducting particles of silicon carbide having a particlesize of 400 to 600 mesh and a binder for said semicon- 5 6 ductingparticles, said binder comprising glass havinga 7. An electric insulatoras claimed in claim 6 fusion temperature of less than 850C, said coatingwherein: being distributed over the surface of said insulating saidbinder comprises PbO, A1 B 0 MgO, and body. LiF.

2. The insulator of claim 1 wherein said insulating 5 8. An electricinsulator as claimed in claim 6 body is comprised of ceramic materialand said ceramic wherein:

coating and said binder being fired onto said insulating said bindercomprises a glass having substantially a body at a temperature of lessthan 800C for approxipercent composition by weight of 22.0 PbO, 616mately 15 minutes. A1 0 65.3 B 0 2.6 MgO, and 3.4 LiF.

3. The insulator of claim 1 wherein a glaze is applied 10 9. An electricinsulator as claimed in claim 5 wheover said insulating body, and saidceramic coating is rein:

fired onto said glaze. said binder comprises glass having a fusiontempera- 4. The insulator of claim 1 wherein a glazed layer is ture ofapproximately 800C. applied over said ceramic coating. 10. An electricinsulator as claimed in claim 9 5. An electric insulator as claimed inclaim 1 wherein: wherein: said binder comprises B 0 A1 0 and BaO.

' said binder comprises 40 percent to 70 percent by 11. An electricinsulator as claimed in claim 10 volume of said ceramic coating.wherein: 6. An electric insulator as claimed in claim 5 said bindercomprises a glass having substantially a wherein: percent composition byweight of 54.2 B 0 1 L5 said binder comprises glass having a fusiontempera- M 0 and 34.3 BaO.

ture of approximately 650C.

2. The insulator of claim 1 wherein said insulating body is comprised ofceramic material and said ceramic coating and said binder being firedonto said insulating body at a temperature of less than 800*C forapproximately 15 minutes.
 3. The insulator of claim 1 wherein a glaze isapplied over said insulating body, and said ceramic coating is firedonto said glaze.
 4. The insulator of claim 1 wherein a glazed layer isapplied over said ceramic coating.
 5. An electric insulator as claimedin claim 1 wherein: said binder comprises 40 percent to 70 percent byvolume of said ceramic coating.
 6. An electric insulator as claimed inclaim 5 wherein: said binder comprises glass having a fusion temperatureof approximately 650*C.
 7. An electric insulator as claimed in claim 6wherein: said binder comprises PbO, Al2O3, B2O3, MgO, and LiF.
 8. Anelectric insulator as claimed in claim 6 wherein: said binder comprisesa glass having substantially a percent composition by weight of 22.0PbO, 616 Al2O3, 65.3 B2O3, 2.6 MgO, and 3.4 LiF.
 9. An electricinsulator as claimed in claim 5 wherein: said binder comprises glasshaving a fusion temperature of approximately 800*C.
 10. An electricinsulator as claimed in claim 9 wherein: said binder comprises B2O3,Al2O3, and BaO.
 11. An electric insulator as claimed in claim 10wherein: said binder comprises a glass having substantially a percentcomposition by weight of 54.2 B2O3, 11.5 Al2O3, and 34.3 BaO.